This invention relates to a high-pressure discharge lamp operating circuit, and more particularly to a high-pressure discharge lamp operating circuit wherein a low-frequency AC operation s transferred to a high frequency operation or a DC operation after arc spots have been located at their normal position (i.e. tips of electrodes) at an early stage of operation.
In high-pressure discharge lamps used in the scientific instruments for the field of measurement, it is preferred that no fluctuation or ripple is present in the light output. To this end, the discharge lamps are frequently operated on a high-frequency power source or DC power source. In this case, in order to produce the arc spots at their normal position which are the tips of electrodes, a low-frequency AC operation was performed at an early stage of operation as disclosed in a co-pending U.S. patent application Ser. No. 603,431 filed on Apr. 24, 1984, now U.S. Pat. No. 4,587,460. When the high-pressure discharge lamp is properly operating, arc spots 2 are produced or formed at the tips of electrodes 1, respectively and a discharge plasma 3 is maintained therebetween in an arc tube as shown in FIG. 4A. Further, the shape of the arc spots, which was not considered in the above-referenced patent at all, is such that the arc spots 2, if formed properly, cover the entire tips of the electrodes 1, respectively as shown in FIG. 4B. In this proper state, the discharge plasma 3 is stable. However, it was found that there is a case wherein minute arc spots 2' are formed on portions of the tips of the electrodes 1, as shown in FIG. 4C. The minute arc spots 2' move around on the tips of the electrodes 1, which causes the discharge plasma 3' also to move, resulting in the fluctuation in the light output.
This fluctuation of the light output is a fatal defect when the high-pressure discharge lamp is to be used in the field of measurement, and accordingly, some means for obviating this defect is required.